Insulin-like growth factor binding protein-3 induces senescence by inhibiting telomerase activity in MCF-7 breast cancer cells

Insulin-like growth factor binding protein-3 (IGFBP-3) has been known to inhibit cell proliferation and exert tumor-suppressing effects depending on the cell type. In this study, we aimed to show that IGFBP-3 induces cellular senescence via suppression of the telomerase activity, thereby inhibiting MCF-7 breast cancer cell proliferation. We found that the induction of IGFBP-3 in MCF-7 cells enhanced the loss of cell viability. Flow cytometry revealed a higher percentage of non-cycling cells among IGFBP-3-expressing cells than among controls. IGFBP-3 induction also resulted in morphological alterations, such as a flattened cytoplasm and increased granularity, suggesting that IGFBP-3 induces a senescence-like phenotype. The percentage of IGFBP-3 expressing cells with senescence-associated β-galactosidase activity was 3.4 times higher than control cells. Telomeric repeat amplification and real-time PCR showed that IGFBP-3 decreased telomerase activity by reducing the levels of the RNA component (hTR) and catalytic protein component with reverse transcriptase activity (hTERT) of telomerase in a dose-dependent manner. These results suggest that IGFBP-3 is a negative regulator of MCF-7 breast cancer cell growth by inducing senescence through telomerase suppression.


IGFBP-3 reduces MCF-7 cell viability.
We added ponasterone A to an MCF-7-derived cell line to express endogenous IGFBP-3 in an inducible manner. IGFBP-3 concentrations in the culture medium were increased on days 1, 2, 3, and 5 with the addition of ponasterone A (Fig. 1A, Supplemetary Fig. S1, Table S1). Co-treatment with ponasterone A and siRNA: IGFBP-3 prevented the increase of IGFBP-3 expression at all the indicated times ( Fig. 1A, Supplementary Fig. S1, Table S1). Ponasterone A-induced IGFBP-3 expression resulted in a significant reduction of cell viability, ranging from 41 to 49% on days 3 and 5, when compared to cells without ponasterone A (p < 0.05) (Fig. 1B). This reduction of cell viability was abolished by co-treatment with siRNA:IGFBP-3, but not scrambled siRNA, demonstrating the specific effect of IGFBP-3 on the loss of cell viability (Fig. 1B).

Effect of IGFBP-3 on cell viability is mediated by a mechanism other than apoptosis. Previ-
ous studies have reported that IGFBP-3 induces apoptosis in various cell systems [10][11][12][13][14] . Therefore, to determine whether only apoptosis contributed to the loss of cell viability by IGFBP-3, we performed cell viability assays using the pan-caspase inhibitor, z-VAD-fmk. The expression of IGFBP-3 or the addition of TNF-α (30 ng/mL), which is known to activate the caspase system, significantly reduced cell viability compared to control cells (72.8% and 82.0% on day 3, p < 0.01 and p < 0.05, respectively) (Fig. 1C). However, treatment with z-VAD-fmk completely abolished the inhibitory effect of TNF-α (p < 0.05) but partially abolished the inhibitory effect of IGFBP-3, suggesting that IGFBP-3 decreased cell viability by a mechanism other than caspase-mediated apoptosis.

IGFBP-3 increases the fraction of non-cycling cells. Our previous study demonstrated that IGFBP-3
inhibits MCF-7 cell proliferation by inducing G1 cell cycle arrest 17 . Next, to determine whether IGFBP-3 induces senescence, a permanent exit from the cell cycle, we assessed cells ectopically expressing IGFBP-3 for an increase in the frequency of non-cycling cells using flow cytometry ( Fig. 2A). The percentage of non-cycling cells increased from 1.25% (in the absence of ponasterone A) to 4.8% (in the presence of ponasterone A) (p < 0.01) (Fig. 2B). The percentage of non-cycling cells induced by IGFBP-3 was reduced from 4.8% to 2.13% by cotreatment with siRNA:IGFBP-3, but not scrambled siRNA, suggesting that this effect was specifically owing to increased IGFBP-3 expression. These results indicate that the induced expression of IGFBP-3 increases the fraction of non-cycling cells in this cell system, suggesting that IGFBP-3 may induce senescence in some cells.

IGFBP-3 induces a senescence-like phenotype. IGFBP-3 induction resulted in morphological altera-
tions, such as flattened cytoplasm and increased granularity (Fig. 3), comparable to those reported in senescent cells 33 . These findings imply that IGFBP-3 expression induces a senescence-like phenotype. These morphological changes were abolished by siRNA:IGFBP-3, but not z-VAD-fmk or scrambled siRNA, suggesting that the induction of senescence-like phenotype by IGFBP-3 is specific, and apoptosis is not the only mechanism responsible for reduced proliferation in cells ectopically expressing IGFBP-3.

IGFBP-3 increases senescence-associated ß-galactosidase activity.
We determined the percentage of cells containing SA-ß-galactosidase, a marker of senescence, using immunocytochemistry 34 . On day 3, the percentage of IGFBP-3-expressing cells with SA-ß-galactosidase activity was 3.4 times higher than that of control cells (p < 0.01) (Fig. 4). Increased SA-ß-galactosidase activity by IGFBP-3 was abolished by the addition of siRNA:IGFBP-3 by about 43% on day 3 (p < 0.01), whereas treatment with scrambled siRNA had no effect. These results suggest that IGFBP-3 induces senescence.
IGFBP-3 decreases telomerase activity. Next www.nature.com/scientificreports/ significantly decreased telomerase activity compared to cells without ponasterone A (p < 0.01) (Fig. 5), suggesting that decreased telomerase activity in IGFBP-3-induced MCF-7 cells may be one of the mechanisms inducing senescence. The decrease in telomerase activity by IGFBP-3 was abolished by co-treatment with siRNA:IGFBP-3, but not z-VAD-fmk or scrambled siRNA. These results suggest that the modulation of telomerase activity by IGFBP-3 is specific and independent of caspase-mediated apoptosis.

IGFBP-3 decreases reverse transcriptase and RNA components of telomerase in IGFBP-3 transfected MCF-7 cells.
We determined the levels of reverse transcriptase (hTERT) and RNA (hTR) to elucidate the potential mechanism by which IGFBP-3 affects telomerase activity. Telomerase activity decreased in IGFBP-3-transfected MCF-7 cells. However, the addition of siRNA:IGFBP-3 to IGFBP-3 transfected MCF-7 cells did not affect the telomerase activity of the cells (Fig. 6A). In reverse-transcription quantitative PCR, IGFBP-3-transfected MCF-7 cells decreased the mRNA levels of hTERT and hTR (Fig. 6B,C). The addition of siRNA:IGFBP-3 to the IGFBP-3-transfected cells did not decrease the mRNA levels of hTERT and hTR, whereas the addition of scrambled siRNA decreased these mRNA levels ( Fig. 6B,C). These results suggest that IGFBP-3 reduces telomerase activity and alters the level of telomerase components, including reverse transcriptase and RNA.
Increased telomerase activity by overexpression of hTERT in normal breast cells is inhibited by exogenous IGFBP-3, thereby increasing senescence in a dose-dependent manner. Next, we transfected hTERT into MCF-12F human normal breast cells to overexpress hTERT. We determined the mRNA and protein levels of hTERT and telomerase activity and SA β-galactosidase activity after the addition of exogenous IGFBP-3 at various concentrations. Exogenous IGFBP-3 decreased the levels of hTERT mRNA (Fig. 7A) and protein (Fig. 7B, Supplementary Fig. S2) in a dose-dependent manner. Along with this, telomerase www.nature.com/scientificreports/ activity was decreased and SA-ß-galactosidase activity was increased in a IGFBP-3 dose-dependent manner (Fig. 7C,D). These results suggest that IGFBP-3 induces cellular senescence through reducing telomerase activity in a dose-dependent manner.   www.nature.com/scientificreports/

Discussion
The present study revealed that IGFBP-3 decreases cell viability of the MCF-7 human breast cancer cell line via inducing senescence. IGFBP-3 expression resulted in morphological changes typical of senescent cells, such as flattened cytoplasm and increased granularity, as well as a marked increase in the percentage of cells expressing SA-ß-galactosidase. Furthermore, BrdU-Hoechst flow cytometry showed that IGFBP-3 increased the fraction of non-cycling cells, implying that IGFBP-3 induces senescence. Previous research suggests that IGFBP-3 inhibits cell proliferation by inducing apoptosis 13,14 . In the present study, we used a pan-caspase inhibitor (zVAD-fmk) to demonstrate that the inhibitory effect of IGFBP-3 on cell viability could be attributable to another mechanism in addition to caspase-mediated apoptosis. Treatment with a pan-caspase inhibitor (zVAD-fmk) only partially abolished the loss of cell viability effect of IGFBP-3, whereas it abolished the growth-inhibitory effect of TNF-α. These results suggest that IGFBP-3 enhances the loss of cell viability partly by inducing senescence, in addition to inducing caspase-mediated apoptosis in MCF-7 human breast cancer cells, as previously reported 13,14 . Cellular senescence is the phenomenon of diploid cells undergoing a permanent cell cycle arrest that limits their capacity to proliferate 35 . Senescent cells show no response to mitogens and are permanently arrested in the G0/G1 phase of the cell cycle. This permanent growth arrest, resulting from telomeric shortening in each cell division, is referred to as replicative senescence 27,28 . In addition, stress-induced premature or accelerated senescence can be observed in cells exposed to oncogenic mutations, oxidative stress, suboptimal culture conditions, or DNA damaging agents 35,36 . Although stress-induced premature senescence was assumed to be independent of telomere length, the fact that stressors can either accelerate telomere shortening 37 or uncap telomeres 38 suggests that stress-induced premature senescence can also be telomere-dependent, similar to replicative senescence. Recently, the concept of senescence has been extended to post-mitotic cells beyond cell cycle arrest 39 , as most cells require the induction of both the senescence-associated secretory phenotype (SASP) 40 and senescence-associated mitochondrial dysfunction 41 to maintain a prolonged cell cycle arrest. Although SASP facilitates the maintenance and spread of senescence, promoting tissue remodeling and repair 42,43 , it also contributes to tissue dysfunction, age-related tissue degeneration, and tumorigenesis 44 . Furthermore, epigenetic reprogramming and chromatin remodeling occur during senescence and stabilize other senescence processes by promoting mitochondrial dysfunction and SASP production 45,46 . Therefore, cellular senescence is defined as a cellular response to cellular stresses and damage 47 to function as an additional barrier against the accumulation of damaged DNA, resulting in the transformation of cells, and an underlying cause of aging 28 .
IGFBP-3 may be involved in both replicative senescence and stress-induced premature senescence. Previous studies have suggested that IGFBP-3 expression increases with age in human dermal fibroblasts, epithelial cells, and umbilical vein endothelial cells [22][23][24] , as well as in breast cancer cells treated with chemotherapeutic drugs 26 and premature senescent fibroblasts exposed to tert-butylhydroperoxide and ethanol 48 . Furthermore, despite www.nature.com/scientificreports/ the complexity and diversity of senescence, general evidence suggests that p53 and/or pRb proteins, along with their regulators, such as p21, p16, and ADP ribosylation factor (ARF), are involved in senescence progression 36,37 .
Our previous study demonstrated that IGFBP-3 induced G1 cell cycle arrest by inhibiting cyclin D1, cyclin D3, cyclin E, cyclin A, CDK2, CDK4, total and phospho-pRb, and increasing p21 and p16 in MCF-7 breast cancer cells 17 . This study suggests that pRb pathways, at least in part, may be involved in IGFBP-3-induced senescence, as the G1 cell cycle arrest includes senescence responses. The specific molecular mechanism of IGFBP-3-induced senescence remains to be established. Senescence is linked to telomere length and dysfunction. Telomere shortening with each replication cycle results from the inability of DNA polymerase, telomere dysfunction by either a change of telomeric DNA structure or sequence, or depleted and mutated telomere proteins, leading to permanent growth arrest. These mechanisms may reduce the risk of tumor formation 36 . Telomerase, a ribonucleoprotein complex with reverse transcriptase activity, can catalyze the synthesis of telomeric DNA onto the ends of chromosomes, providing unlimited proliferative capacity to the cells to avoid senescence 27,28 . In most normal human somatic cells, telomerase activity is diminished 49 , and the physiological shortening of telomeres in normal cells triggers replicative senescence or cell death 50 . In contrast, in most human cancer cells, the relatively high level of telomerase activity sustains the proliferation of the cancer cells 32 . In addition, a feed-forward transcriptional loop exists between oncogenes and TERT in cancer cells, which is functionally critical for oncogenesis 51 . In the present study, we aimed to determine whether IGFBP-3 modulates telomerase activity in MCF-7 human breast cancer cells during senescence induction. We demonstrated that the expression of IGFBP-3 decreased telomerase activity using TRAP assays. The present study further demonstrated that IGFBP-3 inhibits telomerase activity by modulating the expression of telomerase components, such as hTR and hTERT, at the mRNA and protein levels. In addition, we found that exogenous IGFBP-3 decreases hTERT mRNA and protein levels and telomerase activity in hTERT-overexpressed normal breast cells. Direct telomere shortening by IGFBP-3 was not confirmed in this study. However, reduced TERT has been reported to prevent cell proliferation without affecting telomere length 32 . Therefore, our results suggest that IGFBP-3-induced inhibition of telomerase activity is a potential mechanism www.nature.com/scientificreports/ for IGFBP-3-induced senescence in MCF-7 cells. Furthermore, the mechanism underlying the potential inhibition of telomerase by IGFBP-3 in MCF-7 cells remains elusive. Additionally, post-translational modifications (such as phosphorylation, glycosylation, and acetylation) of IGFBP-3 may influence protein activity, interaction, and localization, thereby functioning as a key regulator for IGFBP-3 action 52 . This study did not investigate the occurrence and identification of post-translational modifications; therefore, the biological consequences of posttranslational modifications on IGFBP-3 should be elucidated in further studies. Notably, inverse correlation results suggested that TERT overexpression upregulated several IGF signaling proteins including IGFBP-3 in skeletal stem cells, whereas telomerase deficiency led to a reduction in IGF signaling proteins in osteoblastic cells 53 . This result suggests that impairment of IGF signaling owing to reduced telomerase activity is one of the causes of age-related reduced osteoblast function and bone formation. This is considered an example of organ aging owing to cellular senescence caused by the physiological shortening of telomeres in normal cells, as mentioned earlier in normal human somatic cells. Importantly, the association and causality between IGFBP-3 and telomerase activity may vary depending on the cell type. Therefore, taking this into consideration, future studies on IGF signaling and telomerase activity, including IGFBP-3, should be conducted and analyzed in various ways. Additionally, it is necessary to investigate whether IGF is involved in the mechanism of reducing telomerase activity by IGFBP-3 in MCF-7 cells, which is a result of this study.
This study demonstrated that IGFBP-3 induces senescence in MCF-7 human breast cancer cells by inhibiting telomerase activity, thereby reducing cell viability. In previous studies, IGFBP-3 has been reported to inhibit cell proliferation through the induction of apoptosis or cell cycle arrest. The demonstration of the involvement of IGFBP-3 in the senescence of breast cancer cells in this study presents further evidence of a potential mechanism for the loss of cell viability effect of IGFBP-3, and a potential therapeutic strategy for cancer treatment.

Establishment of an MCF-7-derived cell line inducibly expressing IGFBP-3. An MCF-7-derived
cell line inducibly expressing IGFBP-3 was established as previously described 14 . A cDNA fragment containing a 1.5-kb human IGFBP-3 (equivalent to nucleotides 674-2191 of the cDNA sequence) was ligated into the pIND expression vector containing five modified ecdysone response elements. This construct was co-transfected with a second plasmid (pVgRXR), expressing the ecdysone receptor (VgEcR) and the retinoid X receptor (RXR) into MCF-7 cells using the FuGENE 6 transfection reagent (Roche, Indianapolis, IN, USA). After 48 h, the cells were split into a selective medium containing G418 (800 µg/mL) for pIND/IGFBP-3 and zeocin (100 µg/mL) for pVgRXR, and the foci were picked to generate a stable cell line expressing pVgRXR and pIND/IGFBP-3. Ponasterone A, an ecdysone analog inducing heterodimers of RXR and VgEcR to bind the hybrid ecdysone response element in the pIND vector, was used to express IGFBP-3 in an inducible manner. MCF-7 cells transfected with pVgRXR were used as a negative control (EcR cells).

hTERT plasmid transfections in human normal breast cells. Human normal breast MCF-12F cells
were purchased from the ATCC. MCF-12F cells were cultured in DMEM:F-12 media with 20 ng/mL epidermal growth factor, 100 ng/mL cholera toxin, 0.01 mg/mL human insulin, 500 ng/mL hydrocortisone, and 5% chelextreated horse serum. Cells were incubated at 37 °C in a humidified incubator with 5% CO 2  Cell viability assay. Cell viability was performed using the CellTiter 96 aqueous one cell proliferation assay kit (Promega, Madison, WI, USA) as per the manufacturer's instructions. Culture media was aspirated, followed by adding 20 µL of CellTiter 96 Aqueous one solution containing 100 µL of culture medium directly to culture wells and incubating for 1 h at 37 °C. The number of viable cells was measured in a 96-well plate reader at 490 nm. We used 50 nM siRNA:IGFBP-3 to suppress the expression of IGFBP-3 or 25 µM Z-VAD-fmk to suppress the overall caspase activity, which was shown to completely inhibit apoptosis in the same cell line in our previous study 14 .

Flow cytometric analysis of non-cycling cells. A modified BrdU-Hoechst quenching technique was
performed to quantify the fraction of non-cycling cells according to a previously described method 56 . Cells were grown in 6-well plates until 60-70% confluent, and then the media were replaced with SFM in the presence or absence of 15  www.nature.com/scientificreports/ and the second laser were tuned to 488 nm (15 mW output) and 360 nm (10 mW output), respectively. The mean percentage of the non-cycling cells was determined. Three independent experiments were completed for this analysis.
Senescence-associated β-galactosidase staining. Senescence-associated ß-galactosidase (SA-ßgalactosidase) staining was performed according to a previously described method 34 . Sub-confluent cells were washed twice in PBS, fixed with 2% formaldehyde/0.2% glutaraldehyde in PBS, and rinsed twice with PBS. The cells were stained with a staining solution (0.1% 5-bromo-4-chloro-3-indolyl-ß-d-galactoside, 40 mM citric acid, sodium phosphate (pH 6.0), 5 mM potassium ferrocyanide, 5 mM potassium ferricyanide, 150 mM NaCl, and 2 mM MgCl 2 ). Then, the cells were incubated for 6 h at 37 °C. The cells were rinsed twice with PBS after incubation. Approximately 1000 cells in each plate were counted at × 200 magnification. The proportion of cells exhibiting a medium-to dark-blue stain, indicative of SA-ß-galactosidase activity, was then scored. Duplicate plates/cell lines were used in each experiment. Three independent experiments were completed.
Senescence β-galactosidase activity. β-Galactosidase activity in senescence was measured using the senescence β-galactosidase activity assay kit (Cell Signaling Technology, Beverly, MA, USA) according to the manufacturer's instructions. Cells were washed with cold 1× PBS. After removal of the wash solution, cold 1× Senescence Cell Lysis Buffer with 1.0 mM PMSF was added. After incubation on ice for 5 min, the cell lysate was centrifuged at 14,000 rpm for 5 min at 4 °C. The supernatant was transferred to a fresh tube, and the total protein concentration of each cell lysate was determined using a protein assay. Fifty microliters of the cell lysate was transferred to a 96-well plate, and 50 μL of freshly prepared 2× assay buffer (10 mM BME in 2× senescence reaction buffer) was added. Preparation of cell lysates. Cell lysates were prepared as described previously with minor modifications 25 .
In brief, confluent cells were washed with cold PBS and then scraped from plates in the presence of cold lysis buffer containing 20 mM Tris (pH 8.0), 1% NP-40, 0.1% sodium dodecyl sulfate (SDS), 150 mM NaCl, 0.5% sodium deoxycholate, and complete protease inhibitor mixture (Boehringer Mannheim). Floating cells in the conditioned media were collected via centrifugation and resuspended in cold lysis buffer. Cell lysates obtained from confluent and floating cells were centrifuged for 15 min at 4 °C to remove cell debris. The aliquots were stored at − 70 °C until further use.
Telomeric repeat amplification protocol assay. Telomerase activity was determined by the telomeric repeat amplification protocol (TRAP) using the TeloTAGGG Telomerase PCR ELISA PLUS kit (Roche, Penzberg, Germany) in accordance with the manufacturer's instructions. Briefly, 2 × 10 6 cells were lysed in 200 µL of ice-cold lysis buffer for 30 min. Then, the lysate was centrifuged at 16,000×g for 20 min at 4 °C, and the supernatants were collected. Control templates were included in the kit, which contained positive-telomerase template DNA with the same sequence as a telomerase product with eight telomeric repeats. Two microliters of each extract (corresponding to 2 × 10 3 cell equivalents) were assayed for telomerase activity in a 50 µL reaction using a modified TRAP assay. Telomeric repeats were added to a biotin-labeled primer during the first telomerasemediated extension reaction. Then, the elongation products were amplified in a PTC-220 DNA Engine Dyad PCR machine (MJ Research, Waltham, MA, USA). An aliquot of the PCR product was denatured, hybridized to a DIG-labeled, telomeric repeat-specific probe, and bound to a streptavidin-coated 96-well plate. Finally, the immobilized PCR product was detected with an anti-DIG-horseradish peroxidase coupled antibody, visualized by a color reaction product using the substrate tetramethylbenzidine, and semi-quantified photometrically at 450 nm using a FLUORstar Optima microplate reader (BMG Labtechnologies, Germany). Negative controls were prepared from each cell extract by heat-inactivating the telomerase for 10 min at 85 °C.
Reverse-transcription quantitative PCR. Total RNA was isolated from the cells using a TRIzol reagent (Invitrogen). Five micrograms of total RNA was subjected to reverse transcription using Superscript TM III first-strand synthesis system (Invitrogen) according to the manufacturer's instructions. Real-time PCR was performed in 20 µL reaction mixtures containing cDNA, TaqMan primer pairs, and TaqMan universal PCR master mix (Applied Biosystems, Foster City, CA, USA). The primer pairs were obtained from Applied Biosystems (Hs00972650 for hTERT, TR-ANY for hTR, and Hs99999903 for β-actin). Amplification was performed in duplicate with the ABI 7300 system (Applied Biosystems) with the following profile: 50 °C for 2 min, 95 °C for 10 min, and 40 cycles of 95 °C for 15 s and 60 °C for 1 min. Gene expression for each sample was expressed as the threshold cycle (Ct) normalized to β-actin (ΔCt). The ΔCt values were compared between samples from ponas- www.nature.com/scientificreports/